An ungrounded power grid without a fixed relation to electric ground of any of its lines may also be designated as an IT (French: isolée terre) grid. In such a power grid, an isolation fault between one of the lines and ground only results in this line being grounded, and the power grid may still be operated. However, such an isolation fault should be detected as soon as possible to be able to repair it before any further isolation fault occurs which would force a shutdown of the power grid.
In an IT grid, an isolation fault with regard to ground may be detected by measuring and evaluating the impedance to ground. The impedance to ground is a complex value which is composed of the ohmic isolation resistance of interest as its real part and of a capacitive reactance to ground as its imaginary part. The capacitive reactance to ground is a result of leakage capacitances of the power grid which are effective with respect to ground.
IT grids are often used in photovoltaic systems. Due to the surface area of their solar modules alone, photovoltaic systems exhibit high leakage capacitances with regard to ground. The value of these leakage capacitances varies considerably when, for example, rain falls on the photovoltaic system. Due to high and at the same time varying leakage capacitances, monitoring an isolation of a photovoltaic system operated as an IT grid with regard to ground becomes quite complicated. Further, voltage variations occurring over the respective photovoltaic generator due to MPP tracking may also cause problems in isolation monitoring of photovoltaic systems.
A method of and an apparatus for monitoring an isolation of an ungrounded power grid are known from DE 25 42 811 B1. Here, a sine-shaped test signal having a frequency in a preferred range from 25 Hz to 500 Hz is applied to a measurement object by means of an AC voltage source. A voltage drop over a resistor connected in series with the AC voltage source is used as a measure for the leakage current caused by the test signal. The leakage current is supplied to a rectifier circuitry operating in phase with the test signal to determine the active current part of the leakage current which is in phase with the test signal. The known method and the known apparatus turn out to be suitable to only a very limited extent for isolation monitoring of a photovoltaic system operated as an IT grid as the active current part of the leakage current is comparatively small and may thus only be determined with a comparatively high error.
A further method of and a further apparatus for monitoring the isolation of an ungrounded power grid are known from DE 38 82 833 D2. Here, it is indicated that, in case of an AC current grid being monitored, the test signal applied to the IT grid should have a frequency below the frequency of the AC current and that the frequency should be so low that the current flowing over the leakage capacitance of the power grid monitored is minimized. For example, a frequency of 10 Hz is mentioned. This reactive current part of the leakage current is then compensated for determining the isolation resistance.
In a method of and an apparatus for monitoring an isolation of ungrounded DC and AC power grids which are known from EP 0 654 673 A1, a test signal having a square wave voltage course is applied to the power grid to be monitored by means of a voltage source. Based on the resulting leakage current, the absolute value of the isolation resistance between the measurement object and ground is determined. Due to using a square wave voltage course, transient effects, particularly the loading of any capacitances with respect to ground in the power grid to be monitored, have to be awaited before the leakage current is measured. Due to this, the measurement of the isolation resistance is very slow when high capacitances are present. One tries to compensate for this by applying the next voltage step to the test signal as soon as the previous transient effects have ended and the leakage current has been measured. With high leakage capacitances, however, a single measurement of the isolation resistance may nevertheless take some time in the order of minutes. This is not sufficient for quickly detecting an isolation fault.
Thus, there still is a need of a robust and quickly responding method of and apparatus for monitoring an isolation of an ungrounded power grid which securely detects occurring isolation faults even when high and particularly when varying leakage capacitances are present.